1. Field of the Invention
This invention pertains to the elimination of at least a portion of the liquid in a layer formed by the depositing, especially through the use of paper producing process, of a liquid compound with a low dry material (solid) content, on a mobile, continuous, liquid-porous support. The invention pertains specifically to the continuous production of a layer, by the depositing, on a mobile, continuous, porous support, of a liquid compound with a low dry material content, especially a liquid compound containing cellulose fibers, with the deposited layer having insufficient mechanical properties, during the liquid extraction phase, to allow the transfer from one support to another at a high speed.
2. Background of the Prior Art
The removal of the liquid portion of a layer with a low solid matter content is a problem which is encountered in various industries. This occurs in the removal of water from sediments, for example, sediments obtained in purification stations or those which one seeks to refine, especially as fertilizers.
This can also be useful in the paper industry, in which a liquid compound with a low paper fiber content is deposited on a mobile, porous cloth, to form a layer from which the liquid portion is subsequently extracted.
Although the invention pertains to various industries, the following description will be especially directed towards the application of the invention to the paper industry. In accordance with the invention, "paper" is defned as any cellulose fiberbased material, in the form of layers or sheets.
In the industrial production of paper, a liquid compound with a low paper fiber content, i.e., a low dry content, is poured on a continuously moving porous cloth. To pass from the liquid layer deposited on the formation cloth to the fibrous solid layer, the liquid is eliminated first by a gravitational dripping step, and/or by a suction through the porous cloth, by pressing between cloths and/or rollers; this operation can take place on the initial cloth, i.e., the layer formation cloth, or on one or several other subsequent cloths. After this, the fibrous layer is removed from the last cloth on which it was deposited so that it can be heated, for example, through contact with hot cylinders, so that more of its liquid content can be removed by evaporation. The elimination of liquid by dripping, then by pressing, then by heating and evaporation, is performed while the fibrous layer or bed is placed on different supports, the passage from the formation support to the subsequent support generally being done while the dry matter content is under 30%. Patent publication GB-A No. 1 389 992 describes such a liquid removal process with passage from one support to another.
In the specific case of low basis-weight papers, i.e., basis weights under about 30 g/m.sup.2, the removal of water by evaporation is effected on a single drying cylinder and the transfer of the sheet of paper from the porous formation cloth to the drying cylinder is performed while the dry matter concentration is about 20% to 30% by weight. This implies that the production capacity of a line for low basis weights is limited by the diameter of the drying cylinder.
Other processes or devices for the removal of a portion of the water from a liquid compound with a low dry matter content, or those which are more concentrated, such as a slurry, are known. Thus, patent publication No. GB-A-2 047 396 describes a drying device for a slurry, by dripping, with suction, followed by evaporation of the water. Water extraction is not done in a continuous and effective manner in this case.
Accordingly, a need continues to exist for an effective method of removing liquid from a liquid compound layer with low solid content.